The present invention is concerned with conjugated organic compounds that can be used in electronic devices. In particular, fused thiophene oligomers or polymers can be used to fabricate thin-film transistors (TFTs) of field-effect transistors (FETs).
Organic semiconductors that consist of conjugated oligomers or polymers are subject of considerable current research interest, due to their fundamental opto-electronic physics and their potential applications in photo-diode (see reference 1), light-emitting devices (LEDs) (see reference 2) and thin film transistors (TFTs) (see reference 3) etc. The judicious choice of conjugated segments and their various combinations to form different length oligomers or polymers allows for sophisticated fine tuning of such molecular semiconductors, giving rise to surprisingly useful properties.
Pioneered by Garnier and his co-workers, oligomers of thiophenes ranging from tetramers to octamers have been explored as active materials for TFTs that hold great promise in the fabrication of smart cards and flexible flat-panel displays (see references 4-6). The field effect charge mobility of thiophene oligomers was found to increase with conjugation length and then level off after the hexamer. Longer conjugation length seems to incur processing and purification problems as well as difficulty to achieve long range molecular ordering, and thus adversely affect field effect mobility and on/off ratio.
Soluble regioregular poly(3-hexylthiophene) has very recently shown high mobility (10xe2x88x925 to 0.045 cm2/Vs) (see reference 7), although its on/off ratio was rather low (10 to 103) for reasons mentioned above. Therefore, the large majority of the past work into organic TFT materials has been concentrated on the hexamer of thiophene (or sexithiophene, a-hexathienyl, a-6T) and its alkylated derivatives. As a result, TFTs with mobilities of 0.03 cm2Vxe2x88x921sxe2x88x921 and on/off ratios of more than 106 were achievable. The direction of high mobility in sexithiophene TFTs has been revealed to be parallel to the substrate and perpendicular to the long axis of the thiophene oligomer, indicating the importance of xcfx80 stacking to the contribution of charge mobility.
Katz et al (see reference 8) have recently successfully explored another high mobility (0.04 cm2Vxe2x88x921sxe2x88x921) organic TFT material, bis(benzodithiophene). The benzofused thiophenes may create greater overlap of the xcfx80-conjugated units though detailed crystalline structure was not given. Commercially available pentacene, a more advanced fused-ring compound, has received much less attention compared with thiophene oligomers in the past, probably because only low mobilities of 0.002 (see reference 9) or 0.009 cm2Vxe2x88x921sxe2x88x921 (see references 10, 11) were achieved.
Pentacene TFTs prepared by molecular beam deposition have recently shown high mobility (0.03 cm2Vxe2x88x921sxe2x88x921). Using purer material and a slower deposition rate, workers raised pentacene TFTs to a record high mobility of 0.3-0.7 cm2Vxe2x88x921sxe2x88x921 (see references 12, 13). The high mobility has been associated both with xcfx80 stacking and the macroscopic single-crystal nature of sublimed pentacene films (see reference 14). Most organic materials suffer from poor xe2x80x9cOn/Offxe2x80x9d characteristics (102-105) and high threshold slopes.
An object of the present invention is to solve the above problems associated with prior art materials and devices. Accordingly, the present invention provides a compound or complex comprising at least two moieties, each moiety being comprised of two or more thiophenes fused directly to each other. In the context of this invention the term moiety is intended to extend to different parts of a single monomeric molecule and also repeating units of a dimer, oligomer or polymer.
The present invention also provides a method for the production of a compound as defined above, which method comprises the coupling of one fused thiophene derivative to another fused thiophene derivative.
The present invention further provides an electric, electronic, or optoelectronic component or device comprising a compound as defined above.